4.7 V Operation of the Cr4+/Cr3+ Redox Couple in Na3Cr2(PO4)2F3

Increasing the energy density of sodium-ion battery systems requires high-voltage cathode materials to compensate for the inherently higher redox potential of the Na/Na+ couple compared to the Li/Li+ couple (difference of 0.3 V). Distinct from the high-voltage (>4.2 V) operation using late transition metals (Co3+/Co2+ or Ni3+/Ni2+) in previously reported polyanionic compounds, here we identify the record-high Cr4+/Cr3+ (3d2/3d3) redox potential in Na3–xCr2(PO4)2F3 (0 < x < 1) at 4.7 V vs Na/Na+. Despite higher d-band position at early 3d transition metal with smaller effective nuclear charge compared to late transition metals, Cr 3d(t2g) in less than half-filled state possesses no energy level increments arising from crystal field splitting and intra-orbital Coulombic penalty, leading to extremely high voltages of the Cr4+/Cr3+ (3d2/3d3) redox couple.